Synthesis, structure and isomerization of arylphosphoranes with anti-apicophilic bonding modes using a novel bidentate ligand with two C2F5groups

Abstract

A series of anti-apicophilic pentacoordinate phosphoranes (with one chelating substituent in an O-equatorial, C-apical bonding mode at pentacoordinated phosphorus atom) bearing a para-substituted aryl group (–C₆H₄(p-X); X = H, CF₃, F, OMe) or a mesityl (2,4,6-trimethylphenyl) group were isolated using a novel bulky bidentate ligand with two C₂F₅groups. These phosphoranes were stable to isomerization at room temperature, and quantitatively converted into the corresponding more stable isomers (O-apical) at elevated temperatures in solution. On the basis of a kinetic study, the free energy of activation (ΔG^‡) of the stereomutation of the O-equatorial mesitylphosphorane to its O-apical isomer was higher than that of the CF₃ derivative by 2.6 kcal mol⁻¹, giving rise to a further example of the steric effect of the C₂F₅group to freeze the isomerization of the pentacoordinate phosphorus compounds. Kinetic measurements of the isomerization of the O-equatorial ortho-unsubstituted derivatives (–C₆H₄(p-X)) to the corresponding O-apical isomers suggested that the O-equatorial isomers were stabilized by the π → σ*_P–O interaction in the ground state.